This invention relates generally to nuclear reactor constructions but is primarily directed to thermal insulation for use in liquid metal coolant of fast breeder nuclear reactor constructions.
In one known construction of liquid metal cooled nuclear reactor a fast breeder fuel assembly is submerged in a pool of liquid metal coolant in a primary vessel which is housed in a concrete vault. The fuel assembly is carried by a strongback and is surrounded by a barrier defining an inner or hot region of the pool and an outer or cool region of the pool. There are coolant pumps in the outer region which circulate coolant upwardly through the fuel assembly by way of the strongback thence to heat exchangers in the hot region which discharge to the cool region. During operation of the reactor the temperature of the hot region is approximately 540.degree. C. and that of the cool region approximately 370.degree. C. The barrier comprises an impermeable wall or inner tank and in order to reduce the complex stresses in the wall due to the temperature differential across it and to reduce temperature degradation of the hot region coolant the inner surface of the wall is clad with thermal insulation. Several forms of thermal insulation have been used or proposed but for various reasons have proved unsatisfactory. One form of insulation comprises spaced sheets of stainless steel defining a radial series of compartments in which liquid coolant will stagnate each sheet comprising a pair of membranes welded together face-to-face in quilted pattern manner but there is difficulty in the pre-operation testing of this material and in use superficial thermal expansion sets up complex stresses in the material. Another form of insulation comprises a layer of stainless steel blocks attached to the wall surface and covered with a stainless steel membrane but this kind of insulation is expensive.
An object of the invention is to provide a nuclear reactor construction in which a wall surface is clad with thermal insulation which is capable of being prefabricated in standardised units and then installed in the construction without substantial need for further fabrication.